1. Field of the Invention
The invention relates to an electric power steering apparatus for assisting steering by conferring torque generated by an electric motor to a steering mechanism.
2. Description of Related Art
Conventionally, an electric power steering apparatus that confers torque generated by an electric motor to a steering mechanism has been employed to assist steering. Such a power steering apparatus uses a three-phase brushless motor as the electric motor and a sine-wave driving method for supplying sine-wave currents of each phase to the electric motor is often adopted as a driving method for the three-phase brushless motor.
According to the sine-wave driving method, torque ripple of the electric motor hardly occurs when the waveform of an induced voltage shapes a sine wave.
However, irregularities in arrangement among circuits for supplying currents of each phase to the electric motor cause the waveform of the induced voltage to be deformed as a higher harmonic wave superimposes the sine wave. In particular, in a case where the electric motor is of a type that a permanent magnet is embedded in a rotor (so-called IPM motor), a distribution of magnetic flux density forms a trapezoidal shape, and the waveform of the induced voltage is thereby deformed significantly. The deformation of the waveform of the induced voltage gives rise to torque ripple of the electric motor, and the torque ripple of the electric motor causes oscillation of the steering wheel, which impairs the steering feeling.
An object of the invention is to provide an electric power steering apparatus capable of suppressing the occurrence of torque ripple of the electric motor and thereby improving the steering feeling.
An electric power steering apparatus of the invention is an electric power steering apparatus for assisting steering by conferring torque generated by an electric motor to a steering mechanism. The apparatus includes: a correction table created based on a result of actual measurement of torque ripple occurring when a predetermined reference current is supplied to the electric motor; a basic command value setting portion for setting a basic command value according to a steering manipulation of a driver; a current command value setting portion for setting a current command value by correcting the basic command value set by the basic command value setting portion with reference to the correction table; and a motor current control portion for controlling a current to be supplied to the electric motor based on the current command value set by the current command value setting portion.
According to the invention, the basic command value set according to a steering manipulation of a driver is corrected with reference to the correction table created based on the result of actual measurement of torque ripple (in such a manner so as to suppress the occurrence of torque ripple). The current command value is updated to the corrected value, according to which a value of a current to be supplied to the electric motor is controlled. Consequently, it is possible to suppress the occurrence of torque ripple of the electric motor, and the satisfactory steering feeling can be thus achieved.
The correction table may be a table that defines a correction ratio of the current command value with respect to a given rotational position of a rotor of the electric motor.
In this case, the current command value setting portion may set, for example, a product of the basic command value set by the basic command value setting portion and the correction ratio read out from the correction table to the current command value. Also, the current command value setting portion may include a correction value setting portion for setting a ripple correction value by multiplying the basic command value set by the basic command value setting portion and a value of the correction ratio read out from the correction table in correspondence with the given rotational position of the rotor of the electric motor minus 1, and an addition portion for adding the ripple correction value set by the correction value setting portion to the basic command value set by the basic command value setting portion and outputting the sum as the current command value.
The above and other objects, features, and advantages of the invention will become more apparent from the following description of embodiments with reference to the accompanying drawings.